## Definition The **extension trifecta** is the framing that three complementary mechanisms — Skills, MCP servers, and Hooks — extend an agentic CLI beyond its built-in capabilities. They are *not* interchangeable; each solves a different problem. ## The Three Mechanisms | Mechanism | Adds… | Best for… | | ------------------------- | ------------------------------------------- | -------------------------------------- | | [[Skill]] | Reusable *procedures* and *domain knowledge*| "When asked to X, do Y the way we do." | | [[Model Context Protocol]] | New *tools* and *data sources* | Talking to systems that aren't files. | | [[Hook]] | *Deterministic* automation around events | Lint on save, block secrets, gate PRs. | ## Decision Rubric | Question | If yes → | | ----------------------------------------------------------------- | -------- | | Is this a **procedure** the model decides *when* to invoke? | Skill | | Is this a **tool / data source** the model calls during a task? | MCP | | Does this need to happen **deterministically** on an event? | Hook | | Is it knowledge that should be **available every session**? | [[AGENTS.md Convention File]] | ## Composition Capabilities often span dimensions. *"Run the data-quality SQL when verifying a migration"*: - **MCP server** exposes the SQL queries. - **Skill** documents the procedure (which queries, what order, how to interpret). - **Hook** fires the procedure after every migration file edit. ## Related - [[Skill]] - [[Model Context Protocol]] - [[Hook]] - [[Building Effective AI Agents]]